What could be causing my dogs coughing?

My american eskimo (12 years old) started with a dry cough a few weeks ago. After a few days we took her to the vet. First they said it was probably allergies, so they gave her an antihistamine and tussigon to help the cough. However neither worked and she continued coughing like before. We took the dog back, got an x-ray and blood work, and they said her heart was slightly enlarged but nothing that indicated it would cause the coughing. the vet though she saw something in the dogs bronchial tubes so she gave us meds for bronchitis. When that didn't do a thing we took the dog back and got meds for heart disease. The dog has been on those for 4 days now and is coughing just as much as before. The vet said shes out of ideas and we need to see a specialist. Which will likely cost me hundreds of more dollars on top of the hundreds ive already spent for zero results. She coughs mostly at night and when shes getting up from laying down. The dog eats fine and is actually at her best when we go for walks. What else could be the problem?